1. Field of the Invention
The present invention relates to an apparatus and method for automatic repeat request. More particularly, the present invention relates to an apparatus and method for selectively discarding a Media Access Control (MAC) Packet Data Unit (PDU) from a retransmission buffer of a broadband wireless communication system.
2. Description of the Related Art
Recently, as the performance of mobile communication terminals has dramatically improved and the data transfer rate of communication networks has increased, a high-speed data communication function is drawing much attention. Particularly, a multi-media service function used by a mobile communication terminal, such as a Digital Multimedia Broadcasting (DMB) service, requires a higher data service rate. Such data communications are carried out by controlling data flows between a terminal and a network (e.g., base station). The terminal sends an Acknowledgement (ACK) in response to packet data received and sets a timeout for the ACK.
To ensure stable transmission under the faster data communication rate of the terminal, the ACK signal should be received from a receiver, which is hereafter referred to as an ARQ-RX and a transmitter, which is hereafter referred to as an ARQ-TX, should operate a timer for the ACK signal. When not receiving the ACK signal during the timer operation, the ARQ-TX performs a retransmission, which is referred to as an Automatic Retransmission reQuest (ARQ).
FIG. 1 illustrates a conventional ARQ reset operation when an ARQ timer expires.
When the ARQ-TX transmits a Protocol Data Unit (PDU) to the ARQ-RX, the ARQ-TX and the ARQ-RX operate their ARQ timers independently of each other. The PDU is a data unit exchanged between peer entities of the same protocol layer. In the downlink, the PDU is a data unit generated for the next lower layer. In the uplink, the PDU is a data unit received from the lower layer.
The PDU blocks are transmitted to the ARQ-RX in an ARQ-TX window 102. The PDU blocks are received from the ARQ-TX in an ARQ-RX window 110.
Herein, the ARQ-TX queues an ACK/Negative ACK (NACK) signal of the PDUs to its retransmission buffer 100 until a response is received from the ARQ-RX. The ARQ-RX stores up to which PDU is received without error.
When the ARQ timer, for example, ARQ_SYNC_LOSS_TIMEOUT expires while waiting for the ACK signal after the PDU transmission, the ARQ-TX sends a ARQ_RESET (0x0) message 104 to the ARQ-RX. Herein, when it is informed of the active data transmission, the ARQ_SYNC_LOSS_TIMEOUT is a maximum time interval allowing the ARQ_TX_WINDOW 102 or the ARQ_RX_WINDOW 110 to sustain the same value before the ARQ-TX and the ARQ-RX declare the synchronization loss.
Upon receiving the ARQ RESET (0x0) 104, the ARQ-RX resets every corresponding ARQ parameter (e.g., initializes the ARQ_RX window 110) and sends a ARQ_RESET (0x1) message 106 to the ARQ-TX. The ARQ-RX waits for the PDU from the ARQ-TX after resetting the ARQ parameters.
Receiving the ARQ_RESET (0x1) message 106 from the ARQ-RX, the ARQ-TX resets the corresponding ARQ parameters to their initial value and empties the retransmission buffer. Next, the ARQ-TX waits for packets from the upper layer (e.g., Transmission Control Protocol (TCP) layer) of the MAC layer and restarts the transmission.
Disadvantageously, the ARQ-TX compromises the MAC PDUs in emptying the retransmission buffer after the ARQ_RESET and the compromised MAC PDUs cause the TCP timeout. As a result, the TCP transmission is affected and the throughput of the TCP transmission is deteriorated.
Accordingly, there is a need for an improved apparatus and method for preventing performance degradation in a broadband wireless communication system.